if you're dealing with storage problems, there's multiple ways to remedy this. you could rent out some cloud storage, buy an external drive, upgrade your existing drive or clean up your existing files with a tool like
windirstat.
for cheap laptops i always recommend looking at companies that refurbish older generation business laptops. i really like thinkpads for their build quality and keyboards, i tend to buy them on eBay but ymmv. however, you do need to do your research to make sure you buy a relatively recent processor as you may find listings of core-i5 or core-i7 processors that seem good but are actually a decade old.
personally i need a desktop computer to be more efficient, if you spend a lot of time computing then i believe you ought to have one. it can be a significant investment because you may need a desk, chair, monitor arm, displays etc. however i think the comfort and efficiency really is worth it. if you want to build your own, check out
pcpartpicker. building your own is fun and not too complicated but you can get lost in the weeds with all the options.
if you're on a tight budget, you should be able to find refurbished business computers too, though make sure you get a good deal and not get a machine that's too old for your needs. most of them can be upgraded with extra memory, storage and so forth.